This invention relates to a pattern forming method, and more particularly to a method of forming patterns by using an exposure energy source such as a KrF excimer laser and far or deep ultraviolet rays in a resist material.
When using an excimer laser (ArF: 193 nm, KrF: 249 nm, XeCl: 308 nm, etc.) or far ultraviolet ray (around 190 to 330 nm) as the exposure source, as the material for resist [Deep Ultra-Violet (DUV) resist], AZ2400 (manufactured by Shipley Co.) and PMMA (polymethylmethacrylate) are proposed as the positive type or PGMA (polyglycidyl methacrylate) and CMS (Chloromethylated styrene; by Toyo Soda) as the negative type, among others. PMMA and PGMA are poor in resistance to dry etching and are very inferior in sensitivity. AZ2400 (which is about 10 times as high as PMMA but requires about 1000 to 2000 mJ/cm.sup.2 (at a film thickness of about 0.5 .mu.m) by KrF laser of 249 nm) is resistant to etching (because of novolak resin) and has the highest sensitivity among the commericially developed DUV resists [about 100 mJ/cm.sup.2 (at a film thickness of about 1.0 .mu.m) by KrF laser of 249 nm], but it is small in the difference in transmission before and after exposure when exposed to DUV light, which shows that the components in the resist that absorb the DUV light are contained initially in a considerable amount.
FIG. 8 shows an ultraviolet spectral curve of exposure with 249 nm laser. Accordingly, when a pattern is formed by DUV light using AZ2400, the light is absorbed in the resist, so that a resist pattern with favorable contrast is not formed (For example, H. Ito et al., Sympo. on VLSI Tech., 1982; K. J. Orvek et al., SPIE, 1986; V. Pol et al., SPIE, 1986).
Referring now to FIG. 9, a resist pattern forming method using the conventional AZ2400 is described below. On a substrate 1, AZ2400 is applied by rotation, and a resist film 3 of 1.5 .mu.m in thickness is obtained (FIG. 9A). By KrF excimer laser light 4 of 249 nm, the resist film 3 is selectively exposed to DUV light 4 by using a mask 5 (FIG. 9B). Finally, by an ordinary alkaline development processing, a resist pattern 3a is obtained (FIG. 9C).
In this conventional AZ2400, however, since the light does not reach as far as the lower surface, the shape of the resist pattern 3a is deteriorated. Thus, in the conventional resist film obtained by using AZ2400 having a large surface absorption of light, it is impossible to obtain a fine pattern shape when a light source of short wavelength such as KrF excimer laser of 249 nm is used for exposure.
One of methods which is capable of solving these problems is shown in commonly assigned U.S. application Ser. No. 81,698 (filed; Aug. 3, 1987). This invention provides a similar but different method to solve these problems.